Minor ULE changes and optimizations

[Harrison Grundy]

In addition to the random() replacement patch, I have two other minor
ULE patches:

https://reviews.freebsd.org/D1970
This fixes the comment in sched_balance_pair to reflect what ULE is
actually doing, and moves the load comparison outside of tdq_lock to
avoid taking the lock unless there is actually a load imbalance
between the two tdqs.

https://reviews.freebsd.org/D1969
This allows a non-migratable thread to pin itself to a CPU if it is
already running on that CPU.

I've been running these patches for the past week or so without issue.
Any additional testing or comments would be greatly appreciated.

--- Harrison